The serial protocols like PCI Express and USB have evolved over the years to provide very high operating speeds and throughput. This evolution has resulted in their physical layer protocol becoming very complex. One of the most essential processes at physical layer is link initialization and training process. In the PCI Express devices, this process establishes many important tasks such as link width negotiation, link data rate negotiation, bit lock per lane, symbol lock/block alignment per lane, etc. All these functions are accomplished by Link Training & Status State Machine (LTSSM), which observes the stimulus from remote link partner as well as the current state of the link, and responds accordingly.

The PCI Express link training state machine has many states, which are further classified into multiple sub-states. Each LTSSM sub-state performs a set of well-defined operations and makes a next state transitions based on meeting required conditions. Even after the link is up, the device may need to change these working parameters of the link at runtime. Sometimes, the device may need to re-establish the link or it can be directed to go into a low-power state.